Evaluation of SARS-CoV-2 nucleocapsid antigen in the blood as a diagnostic test for infection and infectious viral shedding

Abstract Background SARS-CoV-2 nucleocapsid antigen can be detected in plasma, but little is known about its performance as a diagnostic test for acute SARS-CoV-2 infection or infectious viral shedding among non-hospitalized individuals. Methods We used data generated from anterior nasal and blood samples collected in a longitudinal household cohort of SARS-CoV-2 cases and contacts. Participants were classified as true positives if PCR-positive for SARS-CoV-2 and as true negatives if PCR-negative and seronegative. Infectious viral shedding was determined by the cytopathic effect from viral culture. Stratified by seven days after symptom onset, we constructed ROC curves to describe optimized accuracy (Youden’s index), optimized sensitivity, and specificity. Results Of 80 participants, 58 (73%) were true positives while 22 (27%) were true negatives. Using manufacturer’s cutoff of 1.25 pg/ml for evaluating infection, sensitivity was higher from 0-7 days (77.6%; 95% CI: 64-88.2) than 8-14 days (43.2%; 95% CI: 31.1-54.5) after symptom onset; specificity was unchanged at 100% (95% CI: 88.1-100). This test had higher sensitivity (100%; 95% CI: 88.4-100) and lower specificity (65%; 95% CI: 40.8-84.6) for infectious viral shedding as compared with infection, particularly within the first week of symptom onset. Although the presence of N-antigen correlated with infectious viral shedding (r:0.63; p < 0.01), sensitivity still declined over time. Additional cutoffs from ROC curves were identified to optimize sensitivity and specificity. Conclusions We found that this SARS-CoV-2 N-antigen test was highly sensitive for detecting early but not late infectious viral shedding, making it a viable screening test for community-dwelling individuals to inform isolation practices.